Currently, there is a great desire to prepare magnetic nanoparticles. Out of the many types of magnetic nanoparticles, iron oxides, particularly magnetite has attracted considerable attention in recent times. Magnetite (Fe3O4) particles are used in various industrial applications such as magnetic seals in motors, magnetic inks for bank cheques, magnetic recording media and biomedical applications. The latter application can include contrast agents for diagnostics and magnetic field-guided carriers for localizing drugs or radioactive therapeutic systems. Currently, there are several chemical methods available for synthesizing magnetite nanoparticles. However, in general, methods to prepare nanoparticles of uniform and well defined crystallinity are rare. Application performances are enhanced at nanometer levels when production methods provide uniform and well defined particles. Further, the agglomeration of particles should be properly controlled for advanced biomedical applications. Typically, synthetic methods are used to prepare magnetite nanoparticles where co-precipitation of ferrous ion (Fe+2) and ferric ion (Fe+3) with addition of ammonia is generally used. Typically, in these processes the control of pH is very important in controlling the size of nanoparticles. Like many precipitation reactions, the nucleation and growth steps determine the size of nanoparticles. Jeong et al., Nanomagnetite particles prepared under the combined addition of urea and ammonia, Key Engineering Materials, Vols 317-318, (2006), pp. 203-206, have proposed a mechanism of Fe3O4 precipitation through α-FeOOH as an intermediate phase.
Aqueous precipitation methods for magnetite nanoparticle formation are commonly practiced and generally include surfactants and polymers as stabilizers. U.S. Pat. No. 6,962,685 B2 to Sun describes the synthesis of magnetite nanoparticles by co-precipitation of a mixture of Fe+2 and Fe+3 salts in the presence of a strong base. To make stable dispersions of magnetite nanoparticles several stabilizers such as long chain alkyl carboxylic acids and alkyl ammonium cations are used.
Commonly in the preparation of magnetite nanoparticles from the natural ore using chemical methods, the ore is dissolved in strong acids followed by co-precipitation using a base. In the preparation of magnetite nanoparticles from high purity natural ores, physical methods such as wet grinding are highly desirable. Furthermore, wet grinding in the presence of a stabilizer would avoid the use of acids and bases and therefore would be of low cost because it involves a one pot synthetic method leading to stabilized magnetite nanoparticles. In addition, such methods would be less hazardous and lower in carbon foot print.
Since in many cases of nanoparticles the agglomeration of particles should be properly controlled for advanced applications, there is a need for processes to provide well defined nanoparticles without agglomeration.